Will They or Won't They?

I've been waiting for the sheep and goats to start eating the lovely and tenacious thistles in the field. They look good enough to eat but my critters seem to want to eat everything else first. Today, I finally saw them nibbling on the pretty purple flowers, even tasting the prickly parts. They must have very tough mouths. I hope they continue to consume the thistles, however, you can lead a goat to thistles but you can't make it eat!